Switched mode step-up converters are utilized in a variety of electronic equipment in which an output voltage is required and is larger than the input voltage provided.
A step-up converter is used for example in AC/DC or so-called bulk converters in high voltage equipment and DC to DC converters that is utilized oftentimes in portable equipment.
The AC/DC bulk converter is usually provided in power factor-corrected equipment where the AC voltage is fed to a step-up converter and through proper control the input current is made to be nearly sinusoidal and in phase with the input voltage. The portable applications involve running the equipment from a voltage source of limited voltage range such as a battery. Since in most cases a voltage level higher than the voltage available from the batteries is required a step-up converter is used to generate a secondary voltage. This secondary voltage may oftentimes be used to drive displays, disk drives, etc., in computer equipment.
To more particularly understand a prior art conventional step-up converter refer now to FIG. 1. The conventional step-up converter 10 includes an inductor 12, one end of which is coupled to the positive end of the voltage source (V.sub.i) 13, the other end of inductor 12 is coupled to one end of switch 14. The other end of the switch 14 is coupled to the negative terminal of V.sub.i 13. A diode 16 is coupled to the inductor 12. The output of diode 16 is coupled to one end of capacitor 18. The other end of capacitor 18 is coupled to the input in parallel fashion to one end of switch 14. A resistor 20 is coupled in parallel to the capacitor 18. Resistor 20 is a load resistor across which the output voltage is measured. A control system 22 controls the operation of switch 14. The circuit 10 operates in the following manner.
The voltage source Vi 13 represents a voltage source that may be either AC or DC depending upon the application. The inductor 12 repeatedly transfers energy from the input to the output as the switch 14 is opened and closed by control system 22. The control system 22 determines the status of switch 14. The output capacitor 18 reduces the ripple on the output voltage wherein the resistor represents the load. Typically switch 14 may be implemented as a power MOSFET, BJT or some other type of power switching transistor.
This type of converter, although it works well for some applications, has some significant problems. Assuming that the converter is off, that is, the switch 14 is open and the input voltage is 0 initially, after a sufficient period of time the output voltage will also be zero. In this state the converter 10 is completely deenergized. If the input voltage is applied in a step manner such as closing a master switch on an AC box or a portable computer, then the inductor 12 and the capacitor 18 form a resonant circuit. It has been shown that the output voltage will overshoot to substantially twice the input voltage before settling down to its steady state value. This overshoot can cause significant problems in that this increased voltage can cause damage to the components therein.
In addition, a large inrush current will be conducted through inductor 12 which can cause a magnetic flux saturation in the core of the inductor 14. The large inrush current may also lead to the destruction of various components in the converter. Finally, the initial position of switch 14 can complicate matters if switch 14 is closed during start up. The inrush current will build up rapidly in the inductor 12 which could lead to the destruction of both the inductor 12 and the switch 14.
As was mentioned above, if switch 14 is in the open position there is a problem of a voltage overshoot and in-rush current. An additional diode 24 is sometimes added between the input voltage and the capacitor to bypass the flow of the initial charge current of the capacitor through the inductor 12. The addition of the diode 24 does lessen the inductor 12 start up problems, but significantly increases the large in-rush current associated with this type of converter 10. Such large currents can lead to premature and false triggering of circuit breakers or the like which often produce failure of other sensitive loads.
Other problems with large in-rush currents are increased power consumption, component stress, and interaction with other loads connected to the input voltage sources.
Another problem with the above-mentioned prior art converter 10 is that there is no protection for the components if excessive current is presented to the converter 10. This becomes a problem when the load resistor 20 is suddenly reduced due to a short circuit, load damage or a like problem. The step-up converter 10 shows a loop formed by the input voltage source 13, inductor 12, step-up diode 16, and the load resistor 20. Since the load resistor 20 is suddenly reduced, the power is continuously drained from the input voltage source regardless of the condition of switch 14 and the inductor 12 current builds up to dangerous and destructive levels. When the output load resistor 20 is damaged, an error could occur that causes the output to be shorted.
Damage to the inductor 12 is almost certain. In addition, switch 14 will be destroyed if it is turned on after the inductor current has built up to a high level. If the cause of the output overcurrent is a mistake by service personnel or the like, the safety of the service personnel is very important, especially in the AC/DC frontend converter application. In such applications, the output voltage of the step up converter is typically in the 380-400 volt range.
Accordingly what is needed is an improved step-up converter which has soft start capability. That is, have the capability to be turned on at a first voltage level until it reaches a steady state position. It is also important that a converter be provided that does not have the large inrush current problems associated with prior art converters. In another aspect, a step-up converter is needed that has overcurrent protection that can protect the devices of the converter when there is a load failure of some sort. It is also important that such a converter be utilized and have the same characteristics as prior art converters. It is also important that the converter be simple and easily implemented into a variety of electronic applications. The present invention provides a converter that has the above-mentioned features.